They're onstage as villagers one night and as upper-class partygoers the next. Some nights they cover the full moral spectrum, appearing as an angelic host and then as orgiastic demons within the space of a couple of hours.

And they do it all while singing.

The members of the San Francisco Opera Chorus are among the most essential elements of any of the company's productions - a constant source of musical virtuosity and theatrical flair, enriching nearly every performance at the War Memorial Opera House. Whatever the assignment - from 18th century London culturati in Handel's "Xerxes" to crowds of Chinese peasants in John Adams' "Nixon in China" - these artists rise to the occasion.

As Ian Robertson, the company's longtime chorus master, puts it, "They're out every night of the week, performing to the very best of their abilities and with a wonderful esprit de corps. That's half of what makes my job so easy."

Yet precisely because of the group's very reliability and consistency, it can be all too easy to overlook the extent of the contribution that the chorus makes. Happily, this week finds the chorus stepping into the limelight for two undertakings keyed to the composers whose bicentennials are being celebrated this year.

For Wagner's "Flying Dutchman," opening on Tuesday, the men and women will split into two separate groups to portray, respectively, a crew of sailors and the women waiting for them onshore. An even more demanding assignment follows on Friday, when Music Director Nicola Luisotti conducts the Opera Chorus and Orchestra - along with the forces of Naples' Teatro di San Carlo - in the Verdi "Requiem."

Already this season the chorus has filled multiple roles in Boito's "Mefistofele" - including the aforementioned angels and demons, not to mention townsfolk and other celebrants - and attended a garden party in the world premiere of Tobias Picker's "Dolores Claiborne."

These projects and others like them call for a distinctive set of musical skills, related to but different from those employed by the solo artists sharing the stage.

"These are operatic singers, who sing full out with vibrato - but who can then pull right back and sound like a symphony chorus," says Robertson. "What we're looking for is beautiful solo voices that can merge without losing any of their quality."

In part, those requirements have to do with the flexible demands of opera.

"In 'Il Trovatore,' for instance, for the 'Anvil Chorus,' you need a big sound with plenty of vibrato. But then you get to the nuns and you need them to sound like a church choir."

It's also an artistic venue that rewards a certain sort of communal mind-set, says Mary Finch, 51, a soprano and a 15-year veteran of the chorus.

"I take great pride in working with my colleagues and being part of a group. That's my mentality, that's who I am. Sometimes solo singers who join the chorus can have a hard time adjusting."

The relationships among the 48 regular choristers (extra singers are brought in for larger assignments such as "Mefistofele") are strengthened by the long hours and weeks of rehearsals that take place each year.

Robertson begins drilling his charges in March, well before the company's summer season begins in June, and they work simultaneously on all the repertoire for the entire calendar year - a different work schedule from that of nearly any other company, he says.

"In most other opera companies, you're practicing the music for one opera while you're performing another, and then as that one goes into production you start to learn the next opera. But because of the compressed nature of the schedule in San Francisco, we have to have all this learned and memorized and polished before we start performing anything.

"I feel this is an advantage in many ways. These choristers are singing together 26 hours a week, and they begin to hear each other, to blend their voices and personalities until the sound is homogenized. Then when it's time to go into staging rehearsals, they're so used to singing with one another and the music is so deeply in their blood, that it's a relatively straightforward process after that."

Working on an entire season at once also allows the singers to rotate among repertoire, rehearsing three hours of Verdi and then three hours of Wagner.

"In a perfect world you might be able to concentrate on just one opera," said tenor Phillip Pickens, 51, "but the truth is that sometimes if you work on one piece for too long, you end up hitting your head against the wall."

"It's like asking you to hold your arm out straight for 45 minutes," said baritone Frederick Matthews. "It's impossible. But if you can put it down for a while, then you can bring it back easily.

"Each opera has its own tessitura, its own range, and when you do that same one over and over, you push those same buttons and you can really get into trouble. But when you switch from 'Nixon in China' to Handel, your voice just goes 'aah.' "

Execute any tempo

Once the choristers have their music securely learned and memorized, Robertson hands them off to the conductor and director of the respective operas. His goal, he says, is to be sure they're ready to execute any tempo and any phrasing a conductor might require.

But he remains on hand to consult on musical matters and on staging as well - an area where directors are often helped by his expertise.

"In a space like the War Memorial, you have to be very careful about where people are standing. Am I going to have all the tenors together on this side of the stage, or do we want a blended or mixed sound? It depends on the music. Do we want the people who have the bigger voices at the front of the stage? No, we want them a little further upstage.

"I enjoy collaborating with directors - of course, some don't want to collaborate, but a lot of them do. The best is when they say, " 'I'm going to stage this scene, and you let me know if you see anything you want to change.' That's a gift to me."

Robertson, 65, began his career as a pianist in his native Scotland, accompanying rehearsals for the Edinburgh Festival Chorus while pursuing a music degree.

Gregarious career

"I played recitals, and played the Brahms D-Minor Concerto, but I wanted a more gregarious career. I wanted to meet and work with people."

He took a temporary job as assistant chorus master for the Scottish Opera, which soon ballooned into a post as head of music staff. It hooked him on opera permanently.

"It was like a great discipline, touring most of the time," he recalled. "You'd be in Newcastle or Manchester or Cardiff, a week here and a week there, doing brush-up rehearsals in hotel foyers and bars. It was a touring life, which I loved."

But all that came to an end in 1987, when Robertson was invited to join the San Francisco Opera and jumped at the chance. He found a chorus, he says, that was in need of a bit of sprucing up.

"They'd been a little left to their own devices. They were very friendly and very welcoming but over a couple of years I had to reaudition them all and make adjustments, which I did with a lot of respect for people who had spent so long singing in the chorus. These were people with a lot of experience, and I didn't want to lose the chorus' body of experience."

Today, Robertson hears about 100 to 150 auditions for new members each year - even though, he says, the turnover rate for the orchestra is only about one singer a year. He also does a "hearing" for each member of the chorus twice every five years, to check in on the state of their voice.

The choristers themselves have followed different career paths on their way to the San Francisco Opera. Some, like Matthews, began as solo singers. Finch dropped out of music entirely for eight years and sold cars in her hometown in Illinois.

Baritone Michael Bell, 41, who just finished his fifth season with the chorus, was a software engineer until 2004.

"The idea I had of a professional career was, you work your day job and then go to whatever opera house or theater and do your job there," he said. "And if you were lucky you got paid."

For some members of the chorus, the idea of getting paid to do what they love still seems hard to believe.

"It doesn't matter how many times we do 'Butterfly,' it's a different experience every time," said Pickens. "Sometimes I'm onstage, I suddenly think, 'I'm singing "Madama Butterfly"! I love life, I love my job! Thank you, Puccini!' " {sbox}